Method of producing alloy nanoparticles

ABSTRACT

A method of producing a homogenous skutterudite compound, CoSb 3 , having a crystallite diameter of 100 nm or less, by a convenient synthesis process, which is a method of producing CoSb 3 , comprising reducing Co 2+  and Sb 3+  to Co 0  and Sb 0 , respectively, in a solution including a Co-containing compound and an Sb-containing compound using a reducing agent, wherein supplied amount of the Co-containing compound and the Sb-containing compound are adjusted in order to set a ratio of a reduction rate of Co 2+  to Co 0  to a reduction rate of Sb 3+  to Sb 0  to 1:2.9 to 1:3.1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application JP 2017-003334 filed on Jan. 12, 2017 and Japanese Patent Application JP 2017-045989 filed on Mar. 10, 2017, the contents of which are hereby incorporated by reference into this application.

BACKGROUND Field

Exemplary embodiments relate to a method of producing alloy nanoparticles.

Description of Related Art

A thermoelectric conversion material is a semiconductor material that induces direct conversion between thermal energy and electric energy based on two basic thermoelectric effects, which are the Seebeck effect and the Peltier effect.

Examples of such thermoelectric conversion material include a skutterudite compound represented by MX₃ (M: cobalt (Co), rhodium (Rh), iridium (Ir), or the like; X: phosphorus (P), arsenic (As), antimony (Sb), or the like) having a crystal structure illustrated in FIG. 1. Various studies have been made on thermoelectric conversion materials containing a skutterudite compound.

For example, JP Patent Publication (Kokai) No. 2005-325419 A discloses a method of producing a skutterudite compound, CoSb₃, having an average particle size of 2 nm to 100 nm by the hot soap method.

JP Patent Publication (Kokai) No. 2011-184723 A discloses a method of synthesizing alloy nanoparticles of CoSb₃ consisting of Co and Sb using a subcritical or supercritical reaction solvent under high-temperature and high-pressure conditions.

JP Patent Publication (Kokai) No. 2010-114419 A discloses a method of producing a nanocomposite thermoelectric conversion material, in which nanoparticles serving as a dispersed material are dispersed in a mother phase of the thermoelectric conversion material, the material having an interface roughness of 0.1 nm or more in the interface between the mother phase of the thermoelectric conversion material and the nanoparticles serving as a dispersed material.

SUMMARY

As an index for evaluating performance of a thermoelectric conversion material, power factor PF=S²σ or dimensionless figure of merit (sometimes also referred to as “thermoelectric conversion efficiency”) ZT=(PF/κ)×T (where S denotes a Seebeck coefficient, σ denotes conductivity, κ denotes thermal conductivity, and T denotes absolute temperature) is used. In order for a thermoelectric conversion material to have favorable thermoelectric characteristics, ZT is preferably large. In other words, it is preferable that the Seebeck coefficient S and the conductivity σ be high and the thermal conductivity κ be low.

In order to increase conductivity σ, it is preferable to reduce impurities in a thermoelectric conversion material and obtain a single-phase semiconductor crystal having a desired composition ratio.

In order to reduce thermal conductivity κ, it is preferable to reduce a crystal particle size (crystallite diameter) so as to block heat in the nanocrystal interface, for example, as illustrated in FIG. 2.

It is required for a semiconductor material having such thermoelectric conversion performance to be produced by a method with high productivity, economic efficiency, and safety.

The method disclosed in JP Patent Publication (Kokai) No. 2005-325419 A is problematic in that a surfactant used as a dispersant in the hot soap method or impurities derived from the surfactant (hereinafter also referred to as “organic impurities”) remain in the skutterudite compound obtained as a product and cause a decrease in conductivity σ. A method of solving such problem involves a heat treatment of the resulting skutterudite compound at temperatures at which organic impurities are decomposed/evaporated, for example, 700° C. to 800° C., for a long period of time, for example, 3 hours to 30 hours. However, in this method, a heat treatment causes an excessive increase in the crystallite diameter of skutterudite compound particles, which results in an increase in thermal conductivity κ, eventually causing a reduction of thermoelectric conversion performance.

The method disclosed in JP Patent Publication (Kokai) No. 2011-184723 A employs a subcritical or supercritical reaction solvent under high-temperature and high-pressure conditions. In order to bring a reaction solvent into a subcritical or supercritical state, it is necessary to configure a system including a synthesis container to be able to endure high-temperature and high-pressure conditions (temperature: not less than 200° C.; pressure: not less than 4.0 MPa). In view of this, the method disclosed in JP Patent Publication (Kokai) No. 2011-184723 A results in very high cost of equipment, which means economic efficiency is low, and safety must be highly considered as well. Besides, in general, a process that requires high pressure is characterized by low productivity and thus it is not applicable for quantity synthesis. In addition, a continuous system (tubular flow system) method is also problematic in that pipe clogging tends to occur. In this case, maintenance frequency must be increased, resulting in low productivity.

Further, compounds produced in the second step disclosed in JP Patent Publication (Kokai) No. 2010-114419 A include particles composed of a specific constituent element alone and/or compounds having a composition ratio other than a desired composition ratio (hereinafter also referred to as “inorganic impurities”). In order to convert such inorganic impurities into a skutterudite product of interest, it is necessary to conduct a treatment for homogenizing the resulting inorganic impurities via a heat treatment, which is a so-called hydrothermal treatment, at a high temperature of, for example, 240° C. to 380° C. for a long period of time of, for example, 24 hours to 72 hours. A hydrothermal treatment at a high temperature for a long period of time causes an excessive increase in particle size due to thermal diffusion during homogenization. As a result, thermal conductivity κ increases, which eventually causes a reduction of thermoelectric conversion performance. It is therefore necessary to additionally conduct a process control which uniformly disperses very small nanoparticles in a skutterudite base material for compounding.

Therefore, exemplary embodiments relate to providing a method of producing a homogenous skutterudite compound, CoSb₃, having a crystallite diameter of 100 nm or less, that is to say, alloy nanoparticles of a skutterudite compound, CoSb₃, by a convenient synthesis process.

For example, in a method of producing CoSb₃, comprising reducing metal ions to the corresponding metals in a solution including a Co-containing compound and an Sb-containing compound using a reducing agent, supplied amounts of the Co-containing compound and the Sb-containing compound are adjusted to bring a ratio of a reduction rate of Co²⁺ to Co⁰ to a reduction rate of Sb³⁺ to Sb⁰ close to a specified value. X-ray diffraction analysis of the obtained CoSb₃ suggests that a single-phase CoSb₃ skutterudite crystal structure appears and the obtained CoSb₃ has a crystallite diameter of 100 nm or less. Based on the findings, exemplary embodiments are shown below.

For example, exemplary embodiments are as follows.

-   (1) A method of producing CoSb₃, comprising reducing Co²⁺ and Sb³⁺     to Co⁰ and Sb⁰, respectively, in a solution including a     Co-containing compound and an Sb-containing compound using a     reducing agent, wherein supplied amounts of the Co-containing     compound and the Sb-containing compound are adjusted in order to set     a ratio of a reduction rate of Co²⁺ to Co⁰ to a reduction rate of     Sb³⁺ to Sb⁰ to 1:2.9 to 1:3.1. -   (2) The method according to (1), wherein the supplied amounts of the     Co-containing compound and the Sb-containing compound are adjusted     in order to set the ratio of the reduction rate of Co²⁺ to Co⁰ to     the reduction rate of Sb³⁺ to Sb⁰ to 1:3. -   (3) The method according to (1) or (2), wherein at least one weak     reducing agent selected from the group consisting of oxalic acid,     ascorbic acid, and citric acid is used as the reducing agent for     reducing Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively. -   (4) The method according to any one of (1) to (3), which is carried     out at a reaction temperature of 250° C. to 320° C. within a     reaction time of 1 hour to 10 hours. -   (5) The method according to any one of (1) to (4), wherein a total     concentration of [Co²⁺ ] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l. -   (6) The method according to (5), wherein the total concentration of     [Co²⁺] and [Sb³⁺] is 0.350 mol/l to 0.770 mol/l.

According to the method of the exemplary embodiments, particles composed of a specific constituent element alone and/or particles of an undesired composition are not generated, or they would be generated in amounts smaller than conventional amounts, in the obtained particles, and the composition of constituent elements between particles is homogenous. Therefore, there is no need to use a dispersant that may cause organic impurities upon production. Accordingly, there is no need to conduct a treatment for removing organic impurities and/or a hydrothermal treatment at a high temperature for a long period of time. As a result, CoSb₃ obtained according to the method of the exemplary embodiments may not substantially contain organic impurities and inorganic impurities, and it may have a single-phase skutterudite crystal structure, and the crystallite diameter of CoSb₃ particles may be maintained at 100 nm or less. In addition, the crystallite diameter of CoSb₃ particles decreases as the total concentration of [Co²⁺] and [Sb³⁺] increases.

Moreover, according to the method of the exemplary embodiments, there is no need to conduct a hydrothermal treatment. In other words, there is also no need to use synthesis equipment that is configured to endure high pressures. Therefore, the exemplary embodiments that can be carried out using a convenient synthesis equipment and method are excellent in terms of productivity, economy, and safety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a crystal structure of a skutterudite compound MX₃.

FIG. 2 illustrates a crystallite state in a thermoelectric conversion material.

FIG. 3 schematically illustrates exemplary embodiments.

FIG. 4 schematically illustrates exemplary embodiments.

FIG. 5 shows XRD diffraction patterns of powders prepared in Comparative Examples 1 and 2 and Examples 1 and 6.

FIG. 6 shows a relationship of the crystallite diameter of the obtained CoSb₃ particles to the total concentration of [Co²⁺] and [Sb³⁺] for the powders prepared in Examples 1 to 6.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments are described in detail below.

Characteristics of the exemplary embodiments are described herein with reference to the drawings, if appropriate. In the drawings, for the sake of clarification, sizes and shapes of individual portions are emphatically shown, which means that the actual sizes and shapes are not exactly illustrated in the drawings. Accordingly, the scope of the exemplary embodiments is not limited to the sizes and shapes of the individual portions illustrated in the drawings. Note that a method of producing alloy nanoparticles of a skutterudite compound CoSb₃ according to the exemplary embodiments is not limited to the embodiments described below, and therefore, it may be carried out with various modifications and improvements to an extent that allows a person skilled in the art to carry out the exemplary embodiments within a scope that does not deviate from the subject matter of the exemplary embodiments.

As illustrated in FIG. 3, the exemplary embodiments relate to a method of reducing CoSb₃, comprising reducing metal ions to the corresponding metals in a solution including a Co-containing compound and an Sb-containing compound using a reducing agent, in which supplied amounts of the Co-containing compound and the Sb-containing compound are adjusted to bring a ratio of a reduction rate of Co²⁺ to Co⁰ (hereinafter sometimes referred to as “R_(Co)”) to a reduction rate of Sb³⁺ to Sb⁰ (hereinafter sometimes referred to as “R_(Sb)”) (hereinafter sometimes referred to as “R_(Co):R_(Sb)”) close to a specified value.

According to the exemplary embodiments, a Co-containing compound is a compound that is dissolved in a solvent to form Co²⁺. Examples thereof include, but are not limited to, cobalt(II) chloride and organic acid salts of cobalt such as cobalt tartrate, cobalt citrate, and cobalt acetate. A compound that is insoluble in a solvent may be used after being dissolved with an acid, if appropriate. In the exemplary embodiments, cobalt acetate, which is unlikely to cause generation of a by-product other than the skutterudite compound CoSb₃, is preferably used as a Co-containing compound.

According to the exemplary embodiments, an Sb-containing compound is a compound that is dissolved in a solvent to form Sb³⁺. Examples thereof include, but are not limited to, antimony(III) chloride and organic acid salts of antimony such as antimony acetate and potassium antimony tartrate. A compound that is insoluble in a solvent may be used after being dissolved with an acid, if appropriate. In the exemplary embodiments, antimony acetate, which is unlikely to cause generation of a by-product other than the skutterudite compound CoSb₃, is preferably used as an Sb-containing compound.

According to the exemplary embodiments, a solvent used for preparing a solution including a Co-containing compound and an Sb-containing compound is a solvent that stabilizes Co²⁺ and Sb³⁺. Examples that can be used include, but are not limited to, polar solvents such as protonic solvents (e.g., water, alcohol, and polyalcohol) and mixtures thereof. In the exemplary embodiments, tetraethylene glycol is preferably used as a solvent.

In a method of preparing a solution including a Co-containing compound and an Sb-containing compound according to the exemplary embodiments, an order of addition of the respective compounds, a temperature for addition of the compounds, a method of mixing the compounds, time required for mixing, and other conditions are not limited. The compounds are mixed in a solvent so that each compound is uniformly dissolved. For example, in a method of preparing a solution including a Co-containing compound and an Sb-containing compound according to the exemplary embodiments, the preparation may be conducted in the following manner. Tetraethylene glycol as a solvent is added to a reaction container at a room temperature of 1° C. to 30° C. Then, cobalt acetate as a Co-containing compound and antimony acetate as an Sb-containing compound are added in that order during stirring with a stirrer. Mixing is carried out for, for example, 0.2 hours to 1 hour until each compound is dissolved in tetraethylene glycol to form a homogenous solution.

According to the exemplary embodiments, dissolved oxygen levels in a solvent and a solution are not limited. The dissolved oxygen levels are preferably adjusted to a low level of, for example, usually 10 mg/l or less and preferably 5 mg/l or less.

Here, a conventional degassing method can be used for decreasing the dissolved oxygen levels. For example, in the case of conventional degassing method, it is possible, but not limited, to conduct bubbling of a solution or a solvent of interest by an inert gas such as nitrogen gas or argon gas at 1° C. to 30° C. and a flow rate of usually 50 ml/minute to 500 ml/minute and preferably 50 ml/minute to 100 ml/minute.

According to the exemplary embodiments, supplied amounts of a Co-containing compound and an Sb-containing compound are adjusted in order to bring R_(Co):R_(Sb) close to a specified value.

The term “specified value” used herein refers to a composition ratio (molar ratio) of Co to Sb (Co:Sb) of the skutterudite compound CoSb₃ that is a desirable semiconductor in the exemplary embodiments, that is to say, 1:3.

Therefore, according to the exemplary embodiments, supplied amounts of a Co-containing compound and an Sb-containing compound are adjusted in order to set R_(Co):Rs_(b) to 1:2.9 to 1:3.1, preferably 1:2.93 to 1:3.07, more preferably 1:2.96 to 1:3.04, and particularly preferably 1:3.

In the exemplary embodiments, R_(Co) can be expressed by the following equation:

R _(Co) =k _(Co)[Co²⁺]

(where k_(Co), denotes a reaction rate constant for a reaction of reducing Co²⁺ to Co⁰, and [Co²⁺] denotes a concentration of Co²⁺ in a solution).

In the exemplary embodiments, R_(Sb) can be expressed by the following equation:

R _(Sb) =k _(Sb)[Sb³⁺]

(where k_(Sb) denotes a reaction rate constant for a reaction of reducing Sb³⁺ to Sb⁰, and [Sb³⁺] denotes a concentration of Sb³⁺ in a solution).

Based on the above, R_(Co):R_(Sb) can be expressed as follows.

R _(Co) :R _(Sb) =k _(Co)[Co²⁺ ]:k _(Sb)[Sb³⁺]

Therefore, according to the exemplary embodiments, supplied amounts of a Co-containing compound and an Sb-containing compound are adjusted based on the above-mentioned relationship of R_(Co):R_(Sb) in order to set k_(Co)[Co²⁺]:k_(Sb)[Sb³⁺] to 1:2.9 to 1:3.1, preferably 1:2.93 to 1:3.07, more preferably 1:2.96 to 1:3.04, and particularly preferably 1:3.

According to the exemplary embodiments, k_(Co), and k_(Sb) values are calculated by predetermining a relationship between a reduction rate and a concentration of each corresponding metal ion to be supplied. A concentration of a metal ion to be supplied that results in a desirable reduction rate is calculated using the corresponding obtained value.

According to the exemplary embodiments, [Sb³⁺] is usually 0.015 mol/l to 0.1 mol/l, preferably 0.03 mol/l to 0.1 mol/l, and more preferably 0.06 mol/l to 0.1 mol/l, and [Co²⁺] is usually 0.006 mol/l to 0.04 mol/l, preferably 0.012 mol/l to 0.04 mol/l, and more preferably 0.024 mol/l to 0.04 mol/l.

According to the exemplary embodiments, an upper limit of a total concentration of [Co²⁺] and [Sb³⁺] is equivalent to the upper limit of solubility of a Co-containing compound and an Sb-containing compound. The total concentration of [Co²⁺] and [Sb³⁺] is preferably 0.140 mol/l to 0.770 mol/l, and more preferably 0.350 mol/l to 0.770 mol/l. In addition, it is preferable to adjust the total concentration of [Co²⁺] and [Sb³⁺] to more than 0.020 mol/l, e.g., 0.030 mol/l or more.

As a result of adjustment of the supplied amounts of a Co-containing compound and an Sb-containing compound in the manner described above, particles composed of a specific constituent element alone and/or particles of an undesired composition are not generated, or they would be generated in amounts smaller than conventional amounts, in the obtained CoSb₃ particles, resulting in a homogenous composition of constituent elements between particles. In addition, by adjusting the total concentration of [Co²⁺] and [Sb³⁺] to more than 0.020 mol/l, it is possible to suppress generation of particles composed of a specific constituent element alone and/or particles of an undesired composition, which might occur because of reduction of reactivity due to dilution.

In addition, the obtained CoSb₃ particles have a single-phase skutterudite crystal structure, and the crystallite diameter of CoSb₃ particles is 100 nm or less. Further, by increasing the supplied amounts of a Co-containing compound and an Sb-containing compound, which in turn means the total concentration of [Co²⁺] and [Sb³⁺], it is possible to further decrease the crystallite diameter of the resulting CoSb₃ particles. When the total concentration of [Co²⁺] and [Sb³⁺] is set to 0.140 mol/l to 0.770 mol/l, CoSb₃ particles having a crystallite diameter of 75 nm or less can be obtained. Further, when the total concentration of [Co²⁺] and [Sb³⁺] is set to 0.350 mol/l to 0.770 mol/l, CoSb₃ particles having a crystallite diameter of 50 nm or less can be obtained.

According to the exemplary embodiments, by increasing supplied amounts of a Co-containing compound and an Sb-containing compound, it is possible to increase R_(Co) and R_(Sb), thereby increasing productivity. In addition, by increasing the supplied amounts, it is possible to further decrease the crystallite diameter of CoSb₃ particles, thereby making it possible to obtain a material having favorable thermoelectric characteristics.

According to the exemplary embodiments, a reducing agent for reducing Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively, in a solution including a Co-containing compound and an Sb-containing compound is not limited as long as it is a reducing agent that can reduce Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively, in the same solution. Examples thereof include weak reducing agents such as oxalic acid, ascorbic acid, citric acid, and mixtures thereof.

The amount of the reducing agent is one or more molar equivalents and preferably two or more molar equivalents with respect to metal ions in a reaction formula for redox reaction of a Co-containing compound and a reducing agent to be used and a reaction formula for redox reaction of an Sb-containing compound and a reducing agent to be used.

As illustrated in FIG. 4, when a type and an amount of a reducing agent are adjusted in the above manner, Sb³⁺ is not over-reduced to Sb³⁻, allowing efficient formation of alloy nanoparticles of the skutterudite compound CoSb₃ (if, for example, Sb³⁺ is over-reduced to Sb³⁻, Sb³⁻ binds to proton so as to be gasified, resulting in a composition deviation).

According to the exemplary embodiments, a reaction temperature for a reaction of reducing Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively, in a solution including a Co-containing compound and an Sb-containing compound is usually 250° C. to 320° C., preferably 270° C. to 320° C., and more preferably 280° C. to 320° C.

According to the exemplary embodiments, reaction time for a reaction of reducing CO²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively, in a solution including a Co-containing compound and an Sb-containing compound is usually 1 hour to 10 hours and preferably 1 hour to 6 hours.

By controlling the reaction temperature and time in the above manner, it is possible to promote a reaction of reducing Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively, with good efficiency.

According to the exemplary embodiments, a dissolved oxygen level in a solution including a Co-containing compound and an Sb-containing compound used for a reaction of reducing Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively, is preferably adjusted to a low level. For example, the dissolved oxygen level is adjusted to a level of 10 mg/l or less and more preferably 5 mg/l or less.

It is possible to prevent re-oxidation of the reduced Co⁰ and Sb⁰ by decreasing the dissolved oxygen level.

According to the exemplary embodiments, in a method of mixing a solution including a Co-containing compound and an Sb-containing compound and a reducing agent so as to induce a reaction, an order of addition of the solution and the reducing agent is not limited, and they are mixed so that the reducing agent is homogenously mixed in the solution. According to the exemplary embodiments, it is preferable to mix a solution including a Co-containing compound and an Sb-containing compound and a reducing agent at a temperature for inducing a reduction reaction. According to the exemplary embodiments, for example, the reaction is conducted in the following manner. Bubbling of a solution including a Co-containing compound and an Sb-containing compound by argon gas or the like is conducted, a temperature of the resulting mixture is increased to a reaction temperature during stirring with a stirrer, and a reducing agent is slowly added once the temperature reaches the reaction temperature.

Alloy nanoparticles of the skutterudite compound CoSb₃ obtained as a result of the reaction are then filtered and washed with a washing solvent such as water or alcohol, thereby sufficiently removing raw materials and the like. Then, the alloy nanoparticles are dried by air-drying with an inert gas, drying in an inert gas atmosphere, vacuum drying, or the like, and optionally pulverized. For pulverization, conventional pulverization techniques involving the use of a mortar, a hammer mill, a ball mill, a bead mill, a jet mill, a roller mill, and the like can be used regardless of dry or wet methods.

In addition, regarding alloy nanoparticles of the skutterudite compound CoSb₃ produced in the exemplary embodiments, the alloy nanoparticles do not substantially contain organic impurities because a dispersant, such as a surfactant, is not used for production. Further, particles composed of a specific constituent element alone and/or particles of an undesired composition are not generated, or they would be generated in amounts smaller than conventional amounts, in the alloy nanoparticles, and the composition of constituent elements between particles is homogenous. Therefore, there is no need to conduct a treatment for removing organic impurities and/or a hydrothermal treatment at a high temperature for a long period of time.

The exemplary embodiments can be carried out in either a batch system or a continuous system.

According to the exemplary embodiments, a molar ratio of supplied amount of a Co-containing compound to supplied amount of an Sb-containing compound is not necessarily identical to the composition ratio (molar ratio) of Co to Sb in the skutterudite compound CoSb₃ that is a desired semiconductor, that is to say, 1:3. This means that the Co-containing compound or the Sb-containing compound might remain after the reaction in some cases. Such remaining Co-containing compound or Sb-containing compound can be used for the next reaction.

Alloy nanoparticles of the skutterudite compound CoSb₃ produced in the exemplary embodiments have a skutterudite crystal structure illustrated in FIG. 1, in which M denotes Co and Z denotes Sb. It is preferable that the alloy nanoparticles have a peak of the CoSb₃ skutterudite crystal structure alone upon measurement by X-ray diffraction analysis (XRD analysis: Ultima IV (RIGAKU)) (i.e., a single-phase CoSb₃ skutterudite crystal structure appears upon XRD analysis).

Alloy nanoparticles of the skutterudite compound CoSb₃ produced in the exemplary embodiments have a crystallite diameter of 100 nm or less, preferably 90 nm to 75 nm, and more preferably 80 nm to 75 nm when the crystallite diameter is measured using the Scherrer's equation at a peak of 2θ=31.3° (013) under the above XRD analysis conditions. In addition, alloy nanoparticles of the skutterudite compound CoSb₃ produced by increasing the total concentration of [Co²⁺] and [Sb³⁺] in the exemplary embodiments have a crystallite diameter of 100 nm or less, preferably 90 nm to 40 nm, more preferably 80 nm to 40 nm, further more preferably 75 nm to 40 nm, and particularly 50 nm to 40 nm when the crystallite diameter is measured using the Scherrer's formula at the peak of 2θ=31.3° (013) under the above XRD analysis conditions.

The Scherrer's equation used herein is an equation for determining a crystallite diameter (D) and can be expressed by the following equation:

D=Kλ/(β cos θ)

[where K denotes a Scherrer constant, λ, denotes a characteristic X-ray wavelength (CuKα), β denotes a half-value width, and θ denotes a diffraction angle].

Alloy nanoparticles of the skutterudite compound CoSb₃ produced in the exemplary embodiments do not substantially contain organic impurities because a dispersant, such as a surfactant, is not used for production. In addition, XRD analysis results of the corresponding alloy nanoparticles suggest that particles composed of a specific constituent element alone and/or particles of an undesired composition are not generated, or they would be generated in amounts smaller than conventional amounts, in the alloy nanoparticles, and the composition of constituent elements between particles is homogenous. Therefore, the alloy nanoparticles do not substantially contain inorganic impurities. Accordingly, it is possible to prevent the alloy nanoparticles from experiencing a reduction of electric characteristics (output PF) derived from impurities, thereby making it possible to achieve high output PF.

Further, a treatment for removing organic impurities and/or a hydrothermal treatment at a high temperature for a long period of time is not conducted in the exemplary embodiments. Therefore, the crystallite diameter of particles in the resulting alloy nanoparticles is maintained at a low level. Accordingly, it is possible to prevent the alloy nanoparticles from experiencing an increase in thermal conductivity due to an excessive increase in the crystallite diameter, thereby making it possible to achieve further increased output PF.

Therefore, alloy nanoparticles of the skutterudite compound CoSb₃ produced in the exemplary embodiments have significantly improved thermoelectric conversion efficiency ZT and thus they are useful as thermoelectric conversion materials.

Moreover, it is possible to carry out the exemplary embodiments in an atmospheric pressure system using a compact and convenient system composed of inexpensive equipment (e.g., grass equipment). Therefore, the exemplary embodiments are excellent in terms of productivity, economy, and safety.

The method according to the exemplary embodiments can be applied to production of, for example, not only skutterudite compound CoSb₃ (such as (Co, Ni)Sb₃ or (Co, Fe)Sb₃)-based thermoelectric conversion materials but also Bi₂Te₃ (such as (Bi, Sb)₂(Te, Se)₃)-based, PbTe-based, or Zn₄Sb₃-based thermoelectric conversion materials.

EXAMPLES

Exemplary embodiments are more specifically described below with reference to the Examples. However, the scope of the exemplary embodiments is not limited to the Examples.

I. Sample Preparation Comparative Example 1 Production of a Powder in such a Manner as to Adjust a Molar Ratio of Supplied Amount of a Co-Containing Compound to Supplied Amount of an Sb-Containing Compound to 1:2.0 and Set a Ratio of a Reduction Rate of Co²⁺ to Co⁰ to a Reduction Rate of Sb³⁺ to Sb⁰ to 1:2.2

-   (1) 0.017 L of Tetraethylene glycol (Tetra-EG) was added to a     beaker. Further, 0.14 g of cobalt acetate (Co(CH₃COO)₂) as a     Co-containing compound and 0.49 g of antimony acetate (Sb(CH₃COO)₃)     as an Sb-containing compound were added thereto during stirring at     30° C. Stirring was continued for 0.5 hours until each raw material     was uniformly dissolved. Thus, a mixed solution was prepared. -   (2) The mixed solution prepared in (1) was bubbled with argon at a     flow rate of 50 ml/minute at 20° C. to obtain a deoxidized liquid     mixture. -   (3) During bubbling with argon gas, the deoxidized liquid mixture     obtained in (2) was heated to 300° C. and a mixture wherein 0.36 g     of oxalic acid ((COOH)₂) was dissolved in 0.003 L of tetraethylene     glycol was added as a reducing agent. -   (4) In the liquid mixture, to which the reducing agent was added in     (3), a reaction was conducted at 300° C. for 360 minutes during     bubbling with argon. Thus, highly crystalline particles, in which     alloying was promoted, was formed. -   (5) The resulting reaction product containing particles formed     in (4) was filtered and washed with ethanol, and then, the solvent     was dried. Thus, a powder was obtained.

Comparative Example 2 Production of a Powder in Such a Manner as to Adjust a Molar Ratio of Supplied Amount of a Co-Containing Compound to Supplied Amount of an Sb-Containing Compound to 1:3.0 and Set a Ratio of a Reduction Rate of Co²⁺ to Co⁰ to a Reduction Rate of Sb³⁺ to Sb⁰ to 1:3.8

A powder was produced as in the case of Comparative Example 1 except that the supplied amounts of the Co-containing compound and the Sb-containing compound in (1) of Comparative Example 1 were changed.

Example 1 Production of a Powder in Such a Manner as to Adjust a Molar Ratio of Supplied Amount of a Co-Containing Compound to Supplied Amount of an Sb-Containing Compound to 1:2.5 and a Total Concentration of [Co²⁺] and [Sb³⁺] to 0.140 mol/l, and Set a Ratio of a Reduction Rate of Co²⁺ to Co⁰ to a Reduction Rate of Sb³⁺ to Sb⁰ to 1:3.0

A powder was produced as in the case of Comparative Example 1 except that the supplied amounts of the Co-containing compound and the Sb-containing compound in (1) of Comparative Example 1 were changed.

Example 2 Production of a Powder in Such a Manner as to Adjust a Molar Ratio of Supplied Amount of a Co-Containing Compound to Supplied Amount of an Sb-Containing Compound to 1:2.5 and a Total Concentration of [Co²⁺] and [Sb³⁺] to 0.210 mol/l, and Set a Ratio of a Reduction Rate of Co²⁺ to Co⁰ to a Reduction Rate of Sb³⁺ to Sb⁰ to 1:3.0

A powder was produced as in the case of Comparative Example 1 except that the supplied amounts of the Co-containing compound and the Sb-containing compound in (1) of Comparative Example 1 were changed.

Example 3 Production of a Powder in Such a Manner as to Adjust a Molar Ratio of Supplied Amount of a Co-Containing Compound to Supplied Amount of an Sb-Containing Compound to 1:2.5 and a Total Concentration of [Co²⁺] and [Sb³⁺] to 0.350 mol/l, and Set a Ratio of a Reduction Rate of Co²⁺ to Co⁰ to a Reduction Rate of Sb³⁺ to Sb⁰ to 1:3.0

A powder was produced as in the case of Comparative Example 1 except that the supplied amounts of the Co-containing compound and the Sb-containing compound in (1) of Comparative Example 1 were changed.

Example 4 Production of a Powder in Such a Manner as to Adjust a Molar Ratio of Supplied Amount of a Co-Containing Compound to Supplied Amount of an Sb-Containing compound to 1:2.5 and a Total Concentration of [Co²⁺] and [Sb³⁺] to 0.490 mol/l, and Set a Ratio of a Reduction Rate of Co²⁺ to Co⁰ to a Reduction Rate of Sb³⁺ to Sb⁰ to 1:3.0

A powder was produced as in the case of Comparative Example 1 except that the supplied amounts of the Co-containing compound and the Sb-containing compound in (1) of Comparative Example 1 were changed.

Example 5 Production of a Powder in Such a Manner as to Adjust a Molar Ratio of Supplied Amount of a Co-Containing Compound to Supplied Amount of an Sb-Containing Compound to 1:2.5 and a Total Concentration of [Co²⁺] and [Sb³⁺] to 0.630 mol/l, and Set a Ratio of a Reduction rate of Co²⁺ to Co⁰ to a Reduction Rate of Sb³⁺ to Sb⁰ to 1:3.0

A powder was produced as in the case of Comparative Example 1 except that the supplied amounts of the Co-containing compound and the Sb-containing compound in (1) of Comparative Example 1 were changed.

Example 6 Production of a Powder in Such a Manner as to Adjust a Molar Ratio of Supplied Amount of a Co-Containing Compound to Supplied Amount of an Sb-Containing Compound to 1:2.5 and a Total Concentration of [Co²⁺] and [Sb³⁺] to 0.770 mol/l, and Set a Ratio of a Reduction Rate of Co²⁺ to Co⁰ to a Reduction Rate of Sb³⁺ to Sb⁰ to 1:3.0

A powder was produced as in the case of Comparative Example 1 except that the supplied amounts of the Co-containing compound and the Sb-containing compound in (1) of Comparative Example 1 were changed.

II. Sample Measurement

With respect to the powders prepared in Comparative Examples 1 and 2 and Examples 1 to 6 in I. Sample preparation, the crystal states thereof were examined by XRD analysis (apparatus: Ultima IV (RIGAKU)). Note that the crystallite diameters of the powders prepared in Examples 1 to 6 were measured using the Scherrer's equation at the peak of 2θ=31.3° (013).

III. Results

Table 1 and FIGS. 5 and 6 show the results.

TABLE 1 Molar ratio of supplied Produced phase amounts (Co:Sb) ([Co²⁺] + Ratio of Primary Secondary Crystallite [Sb³⁺](mol/l)) reduction rates phase phase diameter (nm) Comparative Co:Sb = 1:2.0 Co:Sb = 1:2.2 CoSb₂ CoSb₃ — Example 1 Comparative Co:Sb = 1:3.0 Co:Sb = 1:3.8 CoSb₃ Sb — Example 2 Example 1 Co:Sb = 1:2.5 (0.140) Co:Sb = 1:3.0 CoSb₃ — 75.0 (≤75 nm) Example 2 Co:Sb = 1:2.5 (0.210) Co:Sb = 1:3.0 CoSb₃ — 67.2 (≤75 nm) Example 3 Co:Sb = 1:2.5 (0.350) Co:Sb = 1:3.0 CoSb₃ — 47.4 (≤50 nm) Example 4 Co:Sb = 1:2.5 (0.490) Co:Sb = 1:3.0 CoSb₃ — 49.2 (≤50 nm) Example 5 Co:Sb = 1:2.5 (0.630) Co:Sb = 1:3.0 CoSb₃ — 46.4 (≤50 nm) Example 6 Co:Sb = 1:2.5 (0.770) Co:Sb = 1:3.0 CoSb₃ — 46.6 (≤50 nm)

FIG. 5(a) is an XRD diffraction pattern of the powder prepared in Comparative Example 1, FIG. 5(b) is an XRD diffraction pattern prepared in Comparative Example 2, FIG. 5(c) is an XRD diffraction pattern of the powder prepared in Example 1, and FIG. 5(d) is an XRD diffraction pattern of the powder prepared in Example 6. Table 1 and FIG. 5 indicate the following. The observed XRD diffraction pattern of the powder prepared in Comparative Example 1 includes not only a peak of the skutterudite compound CoSb₃, but also a peak of CoSb₂. The observed XRD diffraction pattern of the powder prepared in Comparative Example 2 includes not only the peak of the skutterudite compound CoSb₃, but also a peak of Sb. On the other hand, each of the observed XRD diffraction patterns of the powders prepared in Examples 1 to 6 includes only the peak of the skutterudite compound CoSb₃.

FIG. 6 shows a relationship of the crystallite diameter of the obtained CoSb₃ particles to the total concentration of [Co²⁺] and [Sb³⁺] for the powders prepared in Examples 1 to 6.

Based on Table 1 and FIG. 6, it was found that the crystallite diameter of the powder prepared in Example 1 was 75 nm, which is not more than 100 nm. It was also found that as the total concentration of [Co²⁺] and [Sb³⁺] was increased, resulting in increased R_(Co) and R_(Sb), the crystallite diameter of the resulting CoSb₃ particles was decreased. More specifically, as the total concentration of [Co²⁺] and [Sb³⁺] was increased from 0.140 mol/l to 0.350 mol/l, the crystallite diameter of the obtained CoSb₃ particles was gradually decreased from 75 nm. In a case in which the total concentration of [Co²⁺] and [Sb³⁺] was 0.350 mol/l to 0.770 mol/l, the crystallite diameter of the obtained CoSb₃ particles became substantially constant at a level of 50 nm or less. This was probably because as the total concentration of [Co²⁺] and [Sb³⁺] was increased over the course of formation of alloy nanoparticles, many nucleation sites were generated.

The present specification includes contents described in the claims, specification and/or drawings of Japanese Patent Application JP 2017-003334 and Japanese Patent Application JP 2017-045989 to which the present application claims priority.

All publications, patents and patent applications cited in the present description are herein incorporated by reference as they are. 

What is claimed is:
 1. A method of producing CoSb₃, comprising reducing Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively, in a solution including a Co-containing compound and an Sb-containing compound using a reducing agent, wherein supplied amounts of the Co-containing compound and the Sb-containing compound are adjusted in order to set a ratio of a reduction rate of Co²⁺ to Co⁰ to a reduction rate of Sb³⁺ to Sb⁰ to 1:2.9 to 1:3.1.
 2. The method according to claim 1, wherein the supplied amounts of the Co-containing compound and the Sb-containing compound are adjusted in order to set the ratio of the reduction rate of Co²⁺ to Co⁰ to the reduction rate of Sb³⁺ to Sb⁰ to 1:3.
 3. The method according to claim 1, wherein at least one weak reducing agent selected from the group consisting of oxalic acid, ascorbic acid, and citric acid is used as the reducing agent for reducing Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively.
 4. The method according to claim 2, wherein at least one weak reducing agent selected from the group consisting of oxalic acid, ascorbic acid, and citric acid is used as the reducing agent for reducing Co²⁺ and Sb³⁺ to Co⁰ and Sb⁰, respectively.
 5. The method according to claim 1, which is carried out at a reaction temperature of 250° C. to 320° C. within a reaction time of 1 hour to 10 hours.
 6. The method according to claim 2, which is carried out at a reaction temperature of 250° C. to 320° C. within a reaction time of 1 hour to 10 hours.
 7. The method according to claim 3, which is carried out at a reaction temperature of 250° C. to 320° C. within a reaction time of 1 hour to 10 hours.
 8. The method according to claim 4, which is carried out at a reaction temperature of 250° C. to 320° C. within a reaction time of 1 hour to 10 hours.
 9. The method according to claim 1, wherein a total concentration of [Co²⁺] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l.
 10. The method according to claim 2, wherein a total concentration of [Co²⁺] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l.
 11. The method according to claim 3, wherein a total concentration of [Co²⁺] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l.
 12. The method according to claim 4, wherein a total concentration of [Co²⁺] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l.
 13. The method according to claim 5, wherein a total concentration of [Co²⁺] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l.
 14. The method according to claim 6, wherein a total concentration of [Co²⁺] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l.
 15. The method according to claim 7, wherein a total concentration of [Co²⁺] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l.
 16. The method according to claim 8, wherein a total concentration of [Co²⁺] and [Sb³⁺] is 0.140 mol/l to 0.770 mol/l.
 17. The method according to claim 9, wherein the total concentration of [Co²⁺] and [Sb³⁺] is 0.350 mol/l to 0.770 mol/l.
 18. The method according to claim 11, wherein the total concentration of [Co²⁺] and [Sb³⁺] is 0.350 mol/l to 0.770 mol/l.
 19. The method according to claim 13, wherein the total concentration of [Co²⁺] and [Sb³⁺] is 0.350 mol/l to 0.770 mol/l.
 20. The method according to claim 15, wherein the total concentration of [Co²⁺] and [Sb³⁺] is 0.350 mol/l to 0.770 mol/l. 